The present invention relates in general to bias circuits and, more particularly, to bias circuits that provide a constant biasing voltage regardless of the operating point of the transistor being biased.
In mobile applications, electronic equipment is required to operate in a stable manner over a fairly large temperature range (e.g., xe2x88x9240xc2x0F. to 100xc2x0 F.). Bias circuits are used to provide stable operating points for radio frequency (RF) discrete transistors (referred to herein as xe2x80x9cexternal transistorsxe2x80x9d) associated with mobile and other applications.
The use of these bias circuits eliminates unacceptable fluctuations in the operating point of the external transistor. This technique allows the biased RF discrete transistor (e.g., an NPN bipolar junction transistor (BJT) or an N-channel metal oxide semiconductor (MOS) transistor) to have its emitter (or source) directly grounded and still operate with a stable collector (or drain) current.
Prior art bias circuits can provide a constant output biasing voltage with a low temperature coefficient (TC) only for a narrow range of operating points of the external transistor. If the operating point of the external transistor falls outside a narrow, design-specific range, a stable bias cannot be guaranteed. A particular design of a prior art bias circuit, therefore, cannot be applied to applications having widely differing transistor operating points.
Hence, there is a need for a bias circuit that provides a stable, accurate, and low voltage bias independent of an external transistor""s specific operating point. Further needed is a bias circuit that can provide a low voltage, stable bias over a wide temperature range.